The goals of Project VIII are to investigate the pathogenesis of recurrent varicella-zoster virus (VZV) infections and the reconstitution of VZV immunity after bone marrow transplantation (BMT). These two objectives are linked by the hypothesis that VZV reactivation and dissemination, are preventable if the host response can be restored by immunization. Our recent experiments in SCID-hu mice demonstrate that VZV infects human CD4+ and CD8+ T-cells. This lymphotropism is a critical factor in the pathogenesis of VZV infections after BMT since life-threatening visceral dissemination is mediated by cell-associated viremia. In order to analyze the pathogenic mechanisms of VZV lymphotropism, we will use cosmids to generate recombinant VZV strains with modifications or deletions of VZV glycoprotein genes and investigate alterations in infectivity for T-cells. T-cell tropism will be assessed in vivo using SCID-hu mice and in vitro using primary human T-cells and T-cells lines. T-cell activation enhances susceptibility to VZV infection in vitro and BMT recipients often have activated T-cells. PBMC from BMT recipients will be tested for activation markers that might enhance the risk of cell-associated viremia during VZV reactivation. Correlations will be made between T-cell activation, VZV viremia detected by polymerase chain reaction (PCR) and susceptibility of T-cells from BMT patients to VZV infection in vitro. Whether VZV infection of T-cells is inhibited by monoclonal antibodies to T-cell antigens will be used to identify possible virus receptors. Our studies of VZV immunity will determine whether immunization of autologous BMT recipients with inactivated varicella vaccine before as well as after transplant accelerates VZV immune reconstitution and whether this vaccine regimen protects against VZV viremia and herpes zoster. Immunologic assays will measure CD4+ T-cell recognition of VZV antigen, glycoprotein E and the IE62 and IE63 proteins; VZV-induced IL-2 and IFN-gamma production will document Th1 function and IL-4 and IL-10 release will be markers of Th2 responses. Recovery of antiviral cytotoxic T-cell function will be evaluated. Vaccine study participants will be monitored for VZV viremia by PCR and for herpes zoster; episodes of VZV reactivation will be correlated with immune reconstitution. These studies of VZV pathogenesis and immunity are expected to generate new approaches to the control of VZV reactivation after BMT and will have direct relevance to strategies for optimal reconstitution of host response to other viral pathogens.